Neck-preserving-stem NPS

ABSTRACT

The present invention essentially relates to a hip joint endoprosthesis stem for cement-free or cemented anchoring in bones that is anchored in the femural neck and in the proximal metaphysis and preserves the internal spongiosa and compact structures that reinforce the femur, that gives the design element axial access to the medullary canal, and possesses parabolically curved outer surfaces to optimize the transfer of force to the bone.

BACKGROUND OF THE INVENTION

[0001] Anatomically, there is little correlation between the morphology of the diaphysis and the structure of and loads on the metaphysis. However, the physiological laws of force transfer that dictate that the bone will be most strongly reinforced at the locations where the strongest loads are encountered do apply. The loading of the bone is reflected in its structure. These principles can be used in designing a prosthesis, in particular in taking the prosthesis interface into account. It has been found that the best long-term results are not obtained with a prosthesis that most closely duplicates the compact exterior geometry of the bone (custom-made prostheses), but rather with one that possesses the reinforcing structures of the bone in an optimal manner (Draenert, K., et al., 1999, Manual of Cementing Technique, Berlin, Heidelberg: Springer). The object of this invention is to provide the reinforcing structures in the proximal femur, to apply the load to the bone proximally, and to achieve a maximum degree of rotational stability.

PRIOR ART

[0002] The design of conventional stems of femural components tends to duplicate the frontal projection of the femur. This is true both for cemented components and for uncemented anchored designs: see Charnley, J., (1960), Anchorage of the Femural Head Prosthesis to the Shaft of the Femur, J. Bone and Joint Surg. B42: 28-30, or also Zweymüller, K. A., et al., (1988), Biologic Fixation of a Press-Fit Titanium Hip Joint Endoprosthesis, Clin. Orthop. 235: 195-206. However, it has been found that a substantial torque load results when the heel strikes the ground, when the patient climbs stairs, or, in general, when the hip joint is extended from the flexed position. If one studies the phylogenetic and ontogenetic development of the femural neck, it becomes clear that this “heel-strike phase” causes a large retrotorsional moment to be applied to the neck of the femur. The design of the prosthesis must take this torque into account.

[0003] Thus far, success in applying the force proximally to the metaphysis has only been achieved by using bone cement, as described in Draenert, K., and Draenert, Y., 1992, Forschung und Fortbildung in der Chirurgie des Bewegungsapparates 3: Die Adaption des Knochens an die Deformation durch Implantate [Research and Advances in Locomotor System Surgery 3: Using Implants to Adapt the Bone to Deformations], Munich: Art and Science. In 1986, M. A. R. Freeman asked why the neck of the femur should be resected (Freeman, M. A. R., 1986, Why Resect the Neck?, J. Bone Joint Surg., 68B: 346-349); increased rotational stability was discussed but not implemented in the design, since the importance of the anatomical structures had not yet been recognized. Thus, the resulting stem was straight and did not exhibit right-left symmetry.

[0004] However, it was found that it was precisely the anatomical structures of the proximal femur that were responsible for the bone's rigidity, torsional strength, and ability to withstand high flexural loads. Thus, all of the anatomical structures, right down to the finest structural reinforcement provided by the spongiosa, need to be provided in the highly integrated overall design.

[0005] The following invention reflects this discovery.

DESCRIPTION OF THE INVENTION

[0006] One of the substantive elements of the prosthesis stem is the straight-line opening of the medullary canal through the greater trochanter (FIG. 01/110). The entire neck of the femur (FIG. 01/120) is preserved through the osteotomy. The design of the stem (FIG. 01/100) allows it to be inserted in a straight line, provided that a guide instrument in the opening canal can be inserted into the medullary canal without encountering resistance. This is the case when the opening has been executed correctly along the dorsal wall of the neck in a straight extension of the canal axis through the greater trochanter. The dorsal wall of the neck forms a straight line with the dorsal wall of the medullary canal.

[0007] The design of the prosthesis takes this anatomy into account and utilizes a cylinder (FIG. 03/109) as a design element located around the canal axis (FIG. 01/130) around which the stem is designed to coincide with the anatomical clearances (FIG. 02.1), which results in symmetrically opposite right-left versions (FIG. 02.2).

[0008] This results in a symmetrically opposite cross section (FIG. 03), which has as its most striking element a flat to deep indentation that is located along the dorsal side of the stem (FIG. 03/104) and that extends in a straight line into the tip of the prosthesis.

[0009] This basically produces a convex-concave-convex curve (FIG. 03/109—FIG. 03/104—FIG. 03/110)=S-shaped curve (FIG. 03/108).

[0010] The reconstruction of the center of rotation takes two parameters into account: First, it was found that there is a close relationship between the so-called metaphysis opening plane (Draenert et al., 1999, Manual of Cementing Technique, Berlin, Heidelberg: Springer) and the center of rotation (RZ), namely that the center of rotation is, as a rule, located 25 mm above this plane. Second, it was found that a prosthesis offset—the distance from the design axis or canal axis to the center of rotation of the head of the femur—of 45 mm was too long (Charnley, J., 1989, Low Friction Arthroplasty of the Hip: Theory and Practice, Berlin, Heidelberg, New York: Springer). Based on clinical experience, a lever arm having a 40-mm offset was considered to the optimal. The design is based on this figure (FIG. 01/101).

[0011] This clinical experience also teaches that a physiological center-collum-diaphysis angle of 126° causes long-term anchorages to experience an increased failure rate. An angle of 135° (FIG. 01/102) has proven to be effective over many years, since the force is applied to the tubular portion of the femur bone as a direct compression load. One of the unique characteristics of this prosthesis is that the axis of the cone does not need to be identical to the CCD angle and instead can be steeper by 3° to 15°—as a rule, 5° to 6° (FIG. 01/103), in order to intensify this effect.

[0012] The length of the stem takes the S-shaped curve in the lateral projection into account. It ranges from 14 cm to 22 cm, and is generally about 15 cm in order to achieve reliable blocking between these curvatures (FIG. 0.2/105, FIG. 02.1/106, and FIG. 02.1/107).

[0013] The unique feature of this prosthesis is its straight-line, anatomically symmetrically opposite design: the outer dorsal surface has an undulating shape in the form of a rounded “3,” the halves of which are uneven (FIG. 03/108). This divides the body of the prosthesis into a lateral cylindrical portion (FIG. 03/109) and a neck portion (FIG. 03/110) joined by connecting portion (FIG. 03/111). The dorsal indentation in the cross-section extends in a straight line along the entire length of the stem, although its depth decreases (FIG. 05/112). The cross sections are kidney-shaped but are not uniformly identical (FIG. 05/112).

[0014] The exterior surfaces are also structured to include coaxially aligned longitudinal grooves and ribs, generally in the ventral and dorsal positions. The design curvature is parabolic, resulting in a U-shaped force transfer (FIG. 02.1/114). This parabolic curve is superimposed on the bone structure (FIG. 02.1/115). The curvature of the exterior surfaces defines the load/deflection diagram in the load test (FIG. 06).

[0015] The insertion hole (FIG. 04/201) for positioning the insertion instrument (FIG. 04/140) is located on the extension of the axis, as is a corresponding abutment surface (FIG. 04/202) for the force fit or thread used to fix the position of the insertion instrument with trial prostheses (FIG. 04/203).

[0016] Both structures are located on the prosthesis shoulder (FIG. 04/200), which forms the transition to the cone (FIG. 04/300) that carries the variable head (FIG. 04/150).

EXAMPLE OF THE INVENTION

[0017] Once the hip joint has been exposed via the standard technique of dorsal or lateral access, the head of the femur is removed along the planned height while preserving the femoral neck and taking the respective metaphysis opening plane into account.

[0018] After the socket has been inserted the femur is positioned is such a way that the medullary cavity can be opened.

[0019] Along the extension of the medullary canal axis, the femur is opened at the “knee” of the greater trochanter using a 11.2-mm diameter (Dm) diamond hollow grinding wheel, and the straight guide instrument, having a diameter Dm of 11 mm and a tip that narrows to 6 mm, is inserted (FIG. 05).

[0020] The femoral metaphysis is then ground open using a hollow diamond-faced grinding wheel, and a trial prosthesis is mounted on the insertion instrument (FIG. 04). The tip of the trial prosthesis is carefully guided in via the opening hole, and then driven into the femur metaphysis via gentle hammering. If this is not possible, the medullary canal of the femur is ground open dorsally while carefully preserving the rear wall of the femural neck, so that the cylindrical component of the stem fits in neatly behind the femural neck.

[0021] The size of the prosthesis is measured in two ways: the guide instrument has a scale for measuring length that indicates the size of the prosthesis. The measuring instrument must be securely restrained in the S-shaped curve of the femur. The size can be read at the transition from the neck to the greater trochanter (FIG. 06). A sliding caliper can be used in the sagittal projection to determine the maximum diameter of the inner contour of the neck. The sliding caliper is also used to determine the size of the prosthesis.

[0022] The final prosthesis is driven in in the same fashion until it is anchored in the conical opening in the neck in such a way that it is absolutely tight and secure.

[0023] Then the prosthesis is repositioned using a trial head, and the luxation tendency is determined. If there is no risk of luxation, the final socket insert and the final head can be installed, whereupon the prosthesis is repositioned, and the work is documented using an imaging device. The muscles are then reattached layer by layer, and the wound is closed up.

Neck-Preserving Stem NPS

[0024]FIG. 01 ap view of neck-preserving prosthesis (NPS) having the following characteristics:

[0025]FIG. 01/100 Anatomical stem, right/left symmetry

[0026]FIG. 01/101 Offset

[0027]FIG. 01/102 CCD angle 135°

[0028]FIG. 01/103 Cone alignment

[0029]FIG. 01/110 Opening channel for diamond

[0030]FIG. 01/113 Coaxial longitudinal structures

[0031]FIG. 01/114 U-shape of force transfer: dorsomedial, medial and anteromedial surface, parabolically curved outer surface

[0032]FIG. 01/120 Osteotomy

[0033]FIG. 01/130 Femur canal axis

[0034]FIG. 01/200 Stem-neck-head transition=shoulder

[0035]FIG. 01/300 Cone

[0036]FIG. 02.1 Axial view of an NPS having the following characteristics

[0037]FIG. 02.1/100 Stem

[0038]FIG. 02.1/104 Dorsal indentation, necking, axially to the tip of the stem

[0039]FIG. 02.1/105 Dorsal, proximal blocking

[0040]FIG. 02.1/106 Ventral blocking

[0041]FIG. 02.1/107 Dorsal, distal blocking

[0042]FIG. 02.1/105 to FIG. 02.1/107 Blocking of the shaft in the sagittal projection

[0043]FIG. 02.1/114 Projecting u-shape with parabolic curvature of outer surface

[0044]FIG. 02.1/115 Recessed bone structure along the osteotomy

[0045]FIG. 02.1/200 Shoulder (stem-neck-cone)—transition

[0046]FIG. 02.1/300 Cone

[0047]FIG. 02.2 Right/left opposing symmetry

[0048]FIG. 03 Proximal cross section of stem showing

[0049]FIG. 03/108 Convex-concave-convex dorsal curvature

[0050]FIG. 03/109 Cylindrical lateral portion

[0051]FIG. 03/110 Medial neck portion

[0052]FIG. 03/111 Transition

[0053]FIG. 04 NPS prosthesis showing

[0054]FIG. 04/140 insertion instrument

[0055]FIG. 04/150 Head ball

[0056]FIG. 04/200 Shoulder curvature, shoulder piece

[0057]FIG. 04/201 Insertion hole

[0058]FIG. 04/202 Abutment surface, or threaded hole (203) of trial prosthesis

[0059]FIG. 04/203 Threaded hole of trial prosthesis

[0060]FIG. 04/300 Outer surface of cone, cone

[0061]FIG. 05 NPS prosthesis

[0062]FIG. 05/100 Showing stem and

[0063]FIG. 05/112 Cross sections

[0064]FIG. 06 Load/deflection diagram from load test of NPS in human femur with two different outer surface curvatures 

1. Anatomical femur component of an artificial hip joint for cement-free and cemented implantation characterized by the following elements: the stem of the prosthesis has an indentation on the dorsal side that extends axially in a straight line into the tip of the prosthesis and on the proximal end of the dorsal side engages the bone structures of the femural neck from the transition to the greater trochanter; the dorsal concave indentation divides the stem proximally into two distinct portions: a medial portion that flares out proximally in a conical shape, and a lateral portion that is almost cylindrical axially along the length of the stem; the cross section of the stem has a basic “comma” shape on the proximal end.
 2. The femur component of claim (1) characterized by the following elements: the stem of the prosthesis has an indentation on the dorsal side that extends axially in a straight line into the tip of the prosthesis and on the proximal end of the dorsal side engages the bone structures of the femural neck from the transition to the greater trochanter; the dorsal concave indentation divides the stem proximally into two distinct portions: a medial portion that flares out proximally in a conical shape, and a lateral portion that is almost cylindrical axially along the length of the stem; the prosthesis stem also has an indentation on the ventral side that extends axially in a nearly straight line into the tip of the prosthesis and divides the stem into a lateral, nearly cylindrical portion, and a medial portion that is conically flared on the proximal end.
 3. The femur component of an artificial hip joint for cement-free or cemented implantation characterized by the following elements: the prosthesis stem has an anatomically configured, conical, right/left symmetrically opposite medial portion, and an axially oriented more-or-less cylindrical portion that is securely connected to said medial portion.
 4. The femur component of claims (1) to (3), wherein the proximal cross section is comma shaped—concave on the dorsal side and having a punctiform bulge in the medial area—and it is symmetrically opposite to the right and left.
 5. The femur component of claims (1) to (4), constructed in such a way that the femur stem axis coincides with the femur canal axis and in the frontal plane the collum-center axis describes an angle between 125° and 145°, generally 135°, with the diaphysis axis (CCD angle), and in the axial top view an angle of 5° to 15°, generally 7°, between the diaphysis and the neck axis—a so-called antetorsion angle—is defined.
 6. The femur component of claims (1) to (5), constructed in such a way that the ventral surface is axially convex and on the ventral side it has concave curvature in such a way that the center of curvature is on the ventral side and the radius of curvature continuously decreases in the proximal direction (parabola).
 7. The femur component of claims (1) to (6), constructed in such a way that the medial outer surface is axially convex and is concave along the medial contour, in such a way that the outer-surface center of curvature is medial and its radius decreases continuously in the proximal direction (parabola).
 8. The femur component of claims (1) to (7), constructed in such a way that the dorsal outer surface of the stem is curved about the axis in a convex-concave-convex configuration from the lateral to the medial in the shape of a breaking wave or a rounded “3” having asymmetrical halves and a rounded transition.
 9. The femur component of claims (1) to (8), constructed in such a way that the lateral outer surface is designed to be largely straight and cylindrical.
 10. The femur component of claims (1) to (9), constructed in such a way that the ventral surface and/or medial surface and/or dorsal surface and/or lateral surface is/are structured by means of coaxially oriented longitudinal beads.
 11. The femur component of claims (1) to (10), constructed in such a way that the stem makes a transition to the cone via a shoulder component (head/neck/stem transition), and, as a modular system, the cone can accept various heads, either centrically or eccentrically, and, to accept a tension anchor, the cone has a central hole and/or one or more holes in the shoulder.
 12. The femur component of claims (1) to (11) constructed in such a way that the cone is oriented between 2° and 9°, generally 5°, the CCD angle does not change, and the offset also remains unchanged, the axis of the cone, however, is projected onto the laterodorsal circumference of the compact femur 2 cm-4 cm below the tuberculum innominatum.
 13. The femur component of claims (1) to (12) constructed in such a way that the implant is made of titanium, tantalum, CoCrMo, or an alloy of titanium, tantalum, or stainless steel.
 14. The femur component of claims (1) to (13) constructed in such a way that the surface of the proximal half has a roughness of 50-250 μm, preferably 80-100 μm.
 15. The femur component of claims (1) to (14) constructed in such a way that the tension anchor of claim 11, by which means the femur component is anchored in the femur with a preload in the femur neck, is embodied as a thrust rod in such a way that the thrust rod can be pushed into the head of the hip (thrust direction), but that it pretensions the prosthesis in the bone in the tension direction. 